The global mining industry is under increasing pressure from citizens and governments around the world to shrink its environmental footprint. In addition, mining company shareholders are applying equal pressure for management to increase profits. Currently, the industry relies on concentrating equipment to create a concentrate comprising a substrate or complex mixture of metals and other materials for shipment to a smelter. This process can leave significant amounts of finely ground minerals and/or toxic chemicals in tailings left on the mine site that can leach out into the environment, sometimes for centuries. The tailings are often environmental hazards and costly for mining companies or governments to maintain and/or remediate.
Conventionally, methods for the recovery of different metals and precious metals are entirely different because of the absence of viable technology to efficiently separate precious metals from other metals, as well as metals from one another. As a result, a large number of mining businesses are specifically focused on the recovery of only one specific metal, leading to a loss of other valuable metals even when they are present in significant proportions in the ore or as a by-product of extraction of the main desirable metal.
Smelters are generally used to treat the concentrates produced on mine sites. In the process, the smelters generate significant amounts of airborne pollution comprising greenhouse gases and common air contaminants, as well as toxic smelter slag. Smelters are also expensive to build, at a cost of approximately $1 billion or more for a smelter to treat one metal. In addition, smelters require large amounts of energy to operate, making the ore smelting process energetically expensive. Ore treated in a smelter is sent to a refinery for conversion into a final value added product which in turn creates additional pollution and expense.
The global steel and foundry industries produce approximately 7-8 billion pounds of furnace dust each year. Approximately 22% (1.5-1.8 billion pounds) of the dust is comprised of zinc, with iron and lesser amounts of lead, silver, cadmium, and other metals making up the balance of the waste. The United States Environmental Protection Agency (EPA) classifies the furnace dust as hazardous waste. Worldwide, most of the created furnace dust has been landfilled for more than a century at great economic and environmental cost.
Copper, silver and gold are generally extracted from sulphide ores and are characterized by their unique physico-chemical characteristics and are essential commodities for industrial applications outside of their monetary or decorative value. All three metals are also excellent conductors of electricity. Copper is the third most common metal in use, trailing only iron and aluminium. Copper sulphides, in naturally occurring mineral deposits, are normally found in association with sulphides of iron, nickel, lead, zinc and molybdenum and often contain traces of silver and gold. Chalcopyrite is one of the most common ores from which copper is extracted. Copper has wide-ranging applications in, for example, electrical wires, roofing and plumbing and industrial machinery.
The conventional extractive metallurgical processes for extracting copper generally involve pyrometallurgical methods for recovering copper values from copper sulphides. Known recovery processes mostly involve grinding the ore, froth flotation (which selectively separates minerals from gangue by taking advantage of differences in hydrophobicity) to obtain an ore concentrate, and roasting and reduction with carbon or electrowinning. However, such treatment often entails expensive mining and beneficiation process steps to concentrate the sulphides. In addition, the production of copper employing the known technology from sulphidic copper ores produces large amounts of sulfur dioxide, carbon dioxide and cadmium vapor. Smelter slag and other residues of the process also contain significant amounts of heavy metals. Further, strict adherence to environmental regulations governing mining operations may substantially increase the cost of recovering copper from its ores by conventional processes.
Acidic metal leaching and recovery processes can suffer from an array of impurities in the final product, even after successive refining steps. Similarly, alkaline methods almost always suffer from inefficient kinetics and low loading capacity. Also, precious metal extraction often makes use of cyanidation, creating, using and discarding cyanide, one of the most toxic chemicals ever known to mankind.
U.S. Pat. No. 3,967,957 (Fonseca) describes a method of utilizing aqueous ammonia as an oxidative leachant for the recovery of metal values from sulfide ore or sulfide source material. Fonseca teaches the use of oxygen, air, or oxygen bearing gas as an oxidation agent. Oxygen gas has limited solubility in an aqueous system, increasing the amount of time required for oxidation, which can be burdensome on a large scale or for commercial deployment.
U.S. Pat. No. 5,308,381 (Han et al.) describes ammonia extraction of gold and silver from ores and other materials.
There remains a need for a method for recovering metals from complex substrates.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.